The present invention is concerned with a novel apparatus for refining digested cellulosic pulp.
In pulp processing systems, it is conventional to subject the digested pulp, after dilution with water, to certain screening and refining operations for the purpose of removing undesired coarse or non-digested materials. For example, a continuous digester, although relatively efficient, does not normally reduce 100% of the woodchips entering the digester so as to completely delignify the fibres. Consequently the pulp as discharged from the digester may contain up to 1% of undigested knots and possibly an additional 1% of small matchstick-like undigested particles generally known as "shives". Before being converted to bleached pulp or unbleached papers, it becomes necessary to remove this small percentage of coarse material from the pulp stock. In conventional operations, this is done in a screen room and, as a part of conventional screen room processing, it is necessary to dilute the stock from its normal 10 to 15% by weight consistency as discharged from the digester, to a consistency in the range of 2 to 4%. At these low consistencies, the stock is sufficiently diluted that it is possible to remove the knots and shives from the pulp stock by the use of various screening apparatus after which the pulp is then again rethickened by means of deckers to consistencies in the range of 8 to 15% for storage and subsequent processing, either as unbleached or bleached pulp. Thus, in essence, the operation taking place in the screen room consists of separating the coarse material from the completely defibred pulp. The separated coarse material is then rethickened and refined so as to defibre it and it is then returned again to the beginning of the screening system for further processing while the screened pulp is thickened for storage or other processing or handling. In some cases knots may be handled separately and returned to the digester system.
The screening operation is generally a wasteful one in that it requires the use of large quantities of water and electric power and much machinery to dilute the digested pulp, remove the small percentage of coarse material from the diluted pulp and then to thicken the screened pulp for storage and the separated coarse material for refining and return to the beginning of the screening system. Thus, as an example, a screen room for a 500 to 1000 ton-per-day mill may cost in the vicinity of two to three million dollars.
The present invention is based on the finding that the extensive screen room operations of conventional pulp processing systems can be eliminated or substantially reduced by placing two or more refiners in series at some convenient point in the blow line (i.e., the discharge line) of a continuous digester in which the washing system is integral with the digester so that washed pulp, essentially free from digesting liquor (alkali) can be discharged directly from the digester into the blow line. The pulp, as discharged from the digester into the blow line in a continuous digester of this type, is under relatively high pressure (e.g. 10-20 atmospheres) and in the complete absence of air at high consistency (e.g. 6-15% solids). The invention is based on the finding that this washed, digested material can be fed directly into two or more refiners placed in series in the digester blow line to refine the stock with great efficiency to reduce shives and other coarse material to fibers, without the need for diluting the stock, screening and refining as in conventional operations. The resultant saving, in terms of equipment, water and power, is immediately evident.
Numerous tupes of pulp refiners are known in the art and any of these may be used for present purposes. Broadly speaking, such refiners constitute a grinding apparatus which includes a rotary grinding disc which has an active face opposed by and spaced from a stationary grinding surface. The digested pulp is introduced into the refiner and, as known, passes between the grinding surfaces where mechanical action causes the coarse material therein, e.g. shives, to be sheared into fibers.
For present purposes, it is preferable to arrange the refiners in series so that the first one is a coarse refiner, i.e., the grinding surfaces are relatively widely spaced from each other to give a coarse degree of refining, and the second refiner is a fine refiner where the grinding surfaces are quite close together to complete the refining operation. The grinding surfaces of the first refiners are spaced a distance X from each other, and the grinding surfaces of the second refiner are spaced a distance Y from each other wherein Y is substantially less than X. In some cases it is possible to effectively use only a single refiner in the blow line but where relatively hard pulp stock, i.e., only partially digested stock, is involved, two or more refiners in series are essential for blow line refining.
The duration of the refining operation will vary depending on other factors, e.g. the rate of pulp fed thereto and the degree of refinement desired. It will be appreciated, however, that the timing and other conditions of the operation are such that the pulp moves continuously as a compact, essentially air-free mass or column of digested material from the digester and digester blow line into and through the refiners to the pulp storage tank or other processing or handling phase. The pressure on the pulp in the refiners will vary depending, for example, on their position in the blow line. For example, the pulp pressure may be in the order of 10-20 atmospheres at or near the discharge from the digester and more like 5-10 atmospheres, or even somewhat less further along the blow line towards the pulp storage tank into which the blow line normally discharges .